1. Field of the Invention
The present invention relates to a power semiconductor device, for example, to a vertical power device such as a vertical diode, a vertical transistor, or the like.
2. Background Art
A vertical power device typically has electrodes on the top and bottom surfaces of a chip. In the OFF state of the vertical power device, a minus voltage is applied to the electrode on the top surface, and a plus voltage is applied to the electrode on the bottom surface. In a vertical diode which is an example of the vertical power device, the electrode on the top surface is an anode electrode, and the electrode on the bottom surface is a cathode electrode. In a vertical MOSFET or a vertical IGBT (Integrated Gate Bipolar Transistor) which is another example of the vertical power device, the electrode on the top surface is a source electrode or an emitter electrode, and the electrode on the bottom surface is a drain electrode or a collector electrode.
In the vertical power device, the vertical diode or the vertical transistor is provided in a device part, while a terminal part is provided in the periphery of the device part so as to prevent a voltage from being applied to the side surfaces of the chip. In the terminal part, an electric field is generated not only in the vertical direction but also in the horizontal direction. Therefore, for the terminal part, a terminal length in the range of twice to four times the thickness of a drift layer that retains a breakdown voltage is needed to prevent the electric field in the horizontal direction from decreasing the breakdown voltage. However, if the terminal length is long, the effective area ratio in the chip decreases and the on-resistance or the on-voltage of the chip increases.
In a well-known structure for shortening the terminal length and increasing the effective area ratio, a deep trench is formed so as to reach a high concentration substrate from the surface of a semiconductor layer (see JP-A 2006-41123 (KOKAI), for example). According to this structure, the decrease of the breakdown voltage in the terminal part can be prevented by insulating the device part and the side surfaces of the chip by the deep trench, and the effective area ratio can be improved by shortening the terminal length to nearly the trench width.
However, it takes a long etching time to form the deep trench. As the trench width becomes narrow, the time becomes longer. On the other hand, if the trench width is wide, the volume of the trench becomes large and it becomes difficult to bury an insulator uniformly in the trench. As a result, voids are generated in the insulator, and a leak current caused by degradation of insulation in the terminal part increases.